Nothing to be stressed about really. If they want to see a budget, then you would put one together that shows projected income and outflows, and send it in. Normally, that sort of thing is reserved for the more awkward consulates, like the infamous Wireless Road in Thailand, but they have the right to ask for it anyplace. I'm surprised that they want to see one in North America, but I don't follow the consulates there as closely as I do for Eastern Europe.
For projected income, I always try to anchor a household budget to the ONS numbers because that makes it objective and difficult to argue with. The latest ONS numbers are at
http://www.statistics.gov.uk/downloads/theme_labour/LFSQS_0105.pdfIn this report, you would go to table 35 and select your region in the UK and use the associated number. For example, a woman working in London averages £496 per week gross. Of course, you can top that up with a larger amount based upon your occupation and so on, but in general that's the methodology to use.
Be sure that it has your name and application number on the top, and lists the UK region where you plan to settle. If you took a number from an ONS table, say so in a footnote.
You don't *HAVE* to do it that way! Note that this is strictly a suggestion, there are no rules or guidelines about how to put one together or what format to use. My suggestion is simply an approach that seems to work. Of course you can do anything that makes sense.
BUT, if they asked for it, then you would need to do something rather than to simply ignore it or try to argue with them that they are being unreasonable. If you wanted to go the route that they are being unreasonable, you should get an advisor to make enquiries on your behalf rather than to write to them yourself.